Defense Primer: Under Secretary of Defense for Intelligence and Security [Updated December 13, 2021] [open pdf - 552KB]
From the Document: "The Under Secretary of Defense for Intelligence and Security (USD(I&S)) is a civilian official, appointed by the President and confirmed by the Senate, who reports directly to the Secretary of Defense. The USD(I&S) is the Principal Staff Assistant and advisor to the Secretary and Deputy Secretary of Defense on intelligence, counterintelligence (CI), security, sensitive activities, and other intelligence-related matters. On behalf of the Secretary, the USD(I&S) also exercises authority, direction, and control over Department of Defense (DOD) intelligence and security agencies, field activities, policy, processes, procedures, and products. [...] As the head of the defense intelligence and security enterprise, the USD(I&S) has responsibility for a significant portion of the personnel and funding supporting the Intelligence Community (IC) overall. [...] IC spending is usually understood as the sum of two separate budget programs: (1) the National Intelligence Program (NIP), which covers IC-wide programs, projects, and activities oriented toward the strategic needs of decision-makers; and (2) the MIP [Military Intelligence Program], which funds defense intelligence activities intended to support tactical military operations and priorities. The DNI [Director of National Intelligence] manages the NIP budget through the IC budget process (Intelligence Planning, Programming, Budgeting and Evaluation Process, or IPPBE). [...]  For FY2021, funds appropriated for the NIP and MIP totaled $84.1 billion ($60.8 billion in NIP funds and $23.3 billion in MIP funds).  For FY2022, funds requested for the NIP and MIP totaled $85.6 billion ($62.3 billion for the NIP and $23.3 billion for the MIP)."
CRS In Focus, IF10523
Congressional Research Service: https://crsreports.congress.gov/